Measurement results of measuring devices that illuminate samples, and measure reflection light, transmitted light, and the like from the samples are affected by intensity of illumination light. Therefore, it is required to monitor the intensity of the illumination light in the measuring devices.
For example, in the technology of Patent Literature 1, a light beam emitted from a light source (LED) in a normal line direction is guided to the sample, and becomes the illumination light, and a light beam emitted from the light source in a non-normal line is guided to a photodetector (light-receiving sensor). The intensity of the light beam guided to the photodetector is detected by the photodetector. A detection result of the photodetector is used for correction of a measurement result.